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Asset Impairment Charges and Reversals - Significant Inputs (Details) | 12 Months Ended
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Dec. 31, 2019 uSDPricePerGallon uSDPerMega-wattHour | Dec. 31, 2016 uSDPricePerGallon uSDPerMega-wattHour
Maximum | Mid-Columbia annual average power prices |  |
Disclosure of impairment loss recognised or reversed [line items] |  |
Significant unobservable input, price per megawatt hour | uSDPerMega-wattHour | 42 | 46
Maximum | On-highway diesel fuel on coal shipments |  |
Disclosure of impairment loss recognised or reversed [line items] |  |
Significant unobservable input, fuel price per gallon | uSDPricePerGallon | 2.40 | 2.09
Maximum | Discount rates |  |
Disclosure of impairment loss recognised or reversed [line items] |  |
Significant unobservable input, assets | 0.064 | 0.057
Minimum | Mid-Columbia annual average power prices |  |
Disclosure of impairment loss recognised or reversed [line items] |  |
Significant unobservable input, price per megawatt hour | uSDPerMega-wattHour | 30 | 22
Minimum | On-highway diesel fuel on coal shipments |  |
Disclosure of impairment loss recognised or reversed [line items] |  |
Significant unobservable input, fuel price per gallon | uSDPricePerGallon | 2.35 | 1.69
Minimum | Discount rates |  |
Disclosure of impairment loss recognised or reversed [line items] |  |
Significant unobservable input, assets | 0.052 | 0.054

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- Definition Line items represent concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes of the table. + References No definition available. + Details
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- Definition The value of significant unobservable input used in the measurement of the fair value of assets. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IFRS -Number 13 -IssueDate 2019-01-01 -Paragraph 93 -Subparagraph d -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=13&code=ifrs-tx-2019-en-r&anchor=para_93_d&doctype=Standard -URIDate 2019-03-27 + Details
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